Who told you animals can't be cruel? Of course they can. It's just more familiar to judge in the human psyche.
It'd also be naïve to assume that there's only one reason humans can be cruel, and, further, to assume that cruelty can be defined without context. In the case of child abuse, it could be: An outlet for stress. The result of childhood abuse/trauma. A side-effect of drugs/alcohol. A biological psychological disorder (ie, schizophrenia). A psychological psychological disorder (Munchausen's). A misguided attempt at parenting and discipline. Etc.
So, why are humans cruel? There are any number of explanations, and for any number of situations.